Verified that this is indeed the case. Since we are using strtol() internally here, I would actually expect INF for both your "inf" and "infinity" cases. The strtol() spec says that this should be the result. And a quick local check verifies this, but the others definitely shouldn't be returning INF. It looks like we are not checking the end pointer returned from strtol() at all in any of our conversion functions. We need something like this:
l = strtol(s, &endp, 0);
if (s != endp && *endp == `\0') {
... valid conversion ...
}

This bug has been fixed in CVS.
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Thank you for the report, and for helping us make PHP better.